1. Field of the Invention
This invention relates to improved adjuvant and vaccine compositions, methods for preparing said improved adjuvant and vaccine compositions, and methods of using the improved compositions.
2. Description of the Prior Art
Conventional vaccines have been used for many years to protect humans and animals from a wide variety of infectious diseases. Typically, these conventional vaccines contain one or more antigens which may include an attenuated pathogen, killed pathogen, or an immunogenic component of a pathogen. In some vaccines, the antigen or antigens may be employed alone to elicit protective immune responses. In other vaccines, the antigen or antigens may be employed together with one or more adjuvants to enhance the immunogenicity of an antigen. One such adjuvant known to the art is monophosphoryl lipid A, which is derived from the lipopolysaccharide of Salmonella minnesota R595. It is also known to the art that monophosphoryl lipid A is a lipidic material which spontaneously aggregates with itself in an aqueous environment. Moreover, it is known that the degree of aggregation has an effect on the activity of monophosphoryl lipid A as an immunostimulant in that the aggregated monophosphoryl lipid A is less stimulatory.
Monophosphoryl lipid A is typically obtained as the triethylamine salt in the form of a lyophilized white powder. Being very hydrophobic, the lyophilized monophosphoryl lipid A does not readily form a clear solution when reconstituted with water but instead yields a turbid mixture with visible white particulates of heterogeneous size that settle out and further aggregate upon standing. To make an acceptable aqueous preparation of monophosphoryl lipid A, it is known to suspend the lyophilized monophosphoryl lipid A triethylamine salt at 1 to 2 mg/mL (w/v) in water containing 0.2% triethylamine, to heat the suspension at 65-70.degree. C., and then to sonicate the mixture. The resulting aqueous preparation, slightly opalescent or clear, is an aqueous colloidal suspension. The triethylamine aids in the solubilization of the monophosphoryl lipid A and may be substituted with similar amounts of triethanolamine.
When aqueous preparations of monophosphoryl lipid A prepared as described hereinabove are frozen and then thawed, however, the monophosphoryl lipid A aggregates resulting in a turbid mixture quite similar in appearance to the turbid mixture of monophosphoryl lipid A prior to sonication. Similarly, when an aqueous preparation of monophosphoryl lipid A as described hereinabove is lyophilized and then rehydrated, the result is also a turbid mixture of aggregated monophosphoryl lipid A.